Extreme ultraviolet (EUV) lithography is poised to complement and eventually replace conventional deep ultraviolet (DUV) lithography due to the significantly narrower illumination wavelength (λ=13.5 nm), providing enhanced patterning resolution and lower process complexity, among other benefits. EUV is currently being developed for possible future use in combination with immersion lithography at 32 nm pitch resolution, sometimes referred to as the 7 nm node.
Optical elements used in EUV lithography are based on reflective rather than refractive optics. EUV mirrors consist of alternating layers of material (e.g., Mo and Si), also called the multilayer. EUV photomasks utilize a reflective coating in addition to an EUV absorber material that has been etched (patterned) to represent the intended circuit design. However, current absorbers used in EUV lithography do not absorb all EUV light and, in fact, have a reflectivity of about 1-3%, depending on absorber height. EUV photomasks also exhibit overlapping shots from neighboring fields, which can cause 1.5%-5.0% extra background light on edges and 4.5%-15% on corners of the circuit designs. The wafer critical dimension (CD) impact can be upwards of about 1 nm/%, causing large critical dimension (CD) drop.